Perturbative gravity at null infinity

TL;DR

This work constructs a chiral field theory on the complexified null boundary of asymptotically flat spacetimes that encodes the radiative modes of gravity and its supersymmetric extensions. By realizing the BMS and extended BMS symmetries as worldsheet charges, it derives Ward identities that reproduce Weinberg's soft graviton theorem and its subleading corrections, linking boundary symmetries to bulk scattering data. The authors show that correlators in this boundary theory yield the tree-level S-matrix of N=8 supergravity via the Cachazo–Skinner representation, offering a perturbative holographic-like description on I_C. The approach connects boundary dynamics to ambitwistor/twistor formalisms and underscores the role of soft theorems in a boundary gravitational framework.

Abstract

We describe a theory that lives on the null conformal boundary of asymptotically flat space-time, and whose states encode the radiative modes of (super)gravity. We study the induced action of the BMS group, verifying that the Ward identity for certain BMS supertranslations is equivalent to Weinberg's soft graviton theorem in the bulk. The subleading behaviour of soft gravitons may also be obtained from a Ward identity for certain superrotation generators in the extended BMS algebra proposed by Barnich & Troessaert. We show that the theory computes the complete classical gravitational S-matrix, perturbatively around the Minkowski vacuum.